Fluidized beds have been used in many applications for the purpose of processing and moving one or more materials and have found application in chemical reactions and physical processes requiring heat transfer, mixing of solids as well as gaseous materials, size enlargement, reduction and classification processes; absorption-desorption, heat treating and coating processes. All solid/gas fluid bed systems include an arrangement for introducing the gas and solid material to the system as well as for causing the solid materials to move through the system in a controlled manner. The operability of a fluid bed system for processing solid materials is critically dependent in many instances on the particle size distribution of the solid material being fluidized. Some materials such as soap products have a very broad particle size distribution and also require gentle handling to avoid fines separation during the drying process. That is, some materials such as those prepared for distribution in the form of granules or flakes of soap products tend to fragment to such an extent as they are being dried in a fluid bed system that they produce an undesirable excess of fine particles which may require at least a partial shut-down of the system operation. This problem may be avoided by operating the system under gentle conditions using less gaseous pressure but then heavy particles begin to accumulate on the gaseous fluid distribution plate producing a system malfunction.
An important element in fluid bed systems is the base or bottom plate which is provided with a number of relatively small openings arrayed in a pattern across the width of the plate through which a gaseous medium under pressure is passed for fluidization of a material to be processed in the system. The gas flow openings are generally designed to minimize back flow of the solid material being processed and to distribute the fluidizing gas evenly. One such bottom plate known in the manufacture of fluidized beds as a "GILL PLATE", bottom plate and manufactured by Niro Atomizer of Soborg, Denmark, is so constituted that the gas passing through the bottom plate not only provides a gaseous force for fluidization but also provides a force to cause movement of the material in a desired direction along the bottom plate. The "gill" type bottom plates are constructed having the "gills" distributed in a rectangular pattern with each "gill" providing orifice openings for gas flow as may be required to satisfy fluidization and material transport requirements for specific materials.
Fluid beds used for drying thermally sensitive materials are generally of the plug flow type operated at relatively low temperatures and requiring relatively long material residence times in the equipment to satisfactorily dry the material. Such dryers are usually provided in a folded design providing a serpentine flow path for the material whereby economies of space, gas ducting and thermal energy can be obtained. In a folded or serpentine flow path dryer design the material being processed loops back and forth as it passes through the equipment. Although this design minimizes the space and distances required in operating the equipment, in folded fluid bed dryers using a conventional bottom plate design as shown in FIG. 6, there is a tendency for some materials being processed to stop fluidizing and pile up at the turn around sections between the adjacent chambers thereby requiring the process to be shut down due to malfunction of the equipment. That is, conventional plug flow fluid beds are fitted with bottom plates rectilinear in form and are placed with no particular consideration as to the energy required to maintain the desired fluidization and directional transport of the material being processed in the turn around sections of a folded, plug flow fluid bed.
This invention teaches an improved fluid bed dryer design and more specifically, an improved fluid bed base or bottom plate design wherein each incremental volume of the material being processed adjacent an incremental area of the gas distribution bottom plate is provided with that energy required to maintain the bed in a fluid condition and to transport the material adjacent the plate in a specific desired direction away from that incremental area of the plate and to an area from which additional material is being removed in a specific direction with a minimum loss of fines.
The object of the present invention is to provide an improved fluid bed dryer particularly suited for drying a readily decrepitateable, feed material of broad particle size distribution which is capable of sustained operation without process interruption because of fluidization or material transport failure while continuously producing a product having a minimum of fines.
The object of this invention is attained by providing a bottom plate design which provides continuous process operation and avoids the accumulation of heavy particles in the turn around sections using a minimum of gas flow energy for fluidization and material transport. The present invention avoids the formation and blow off of an undesirable quantity of fine particles by avoiding the use of energy in excess of that required for fluidization and material transport.
A further object of this invention is to provide an improved bottom plate design particularly suited for use in equipment used for pre-drying feed materials prior to final drying.
The improved bottom plate design specifically orients the bottom plate gas flow openings so as to cause the material immediately adjacent the bottom plate to be moved in nominal parallel flow from one area to another with no material accumulation at any area.